ACX has specific instructions about formatting the MP3 files to be submitted, and recommends the fre:ac utility to achieve this. However, it appears that you can achieve the same thing in the Audacity export after installing the LAME encoder. Am I missing something?
Yes you can create ACX compatible MP3s with Audacity (requires Lame to be installed as you rightly said).
However, you should also keep your backup copy as an exported WAV file.
MP3 encoding always reduces the sound quality to some extent. Editing an MP3 and then exporting / encoding to MP3 reduces the quality a little more. It’s similar to making a photocopy of a photocopy - each ‘generation’ is a bit worse than the previous one. Keeping backup copies of the original source material is highly recommended, and should be in WAV format. A backup version of the finished product is also highly recommended, and should be a WAV export of the project (not a copy of the MP3).
Note that the default settings for MP3 export are not compatible with ACX requirements. You must set the “Bit Rate Mode” to “CBR” (constant bit rate).
The bit-rate should be 192 kbps. (This is the Audacity default when CBR is selected).
ACX recordings should also be mono. The MP3 export will automatically be mono if the Audacity project is entirely mono. To ensure that MP3s are always exported as mono, (even if the project contains stereo), you can enable “Force export to mono” in the MP3 settings.
The sample rate of the MP3 will be the same as the Audacity “Project Rate” (lower left corner of the main Window). ACX require this to be 44100.